In an exhaust gas discharged from an internal combustion engine of automobiles, etc., nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbon (HC), etc. are contained, and these substances can be removed by an exhaust gas purification catalyst of oxidizing CO and HC and reducing NOx. As a representative exhaust gas purification catalyst of this type, a three-way catalyst obtained by supporting a noble metal such as platinum (Pt), rhodium (Rh) and/or palladium (Pd), on a porous metal oxide support such as γ-alumina is known.
The metal oxide support may be formed of various materials, but alumina is generally used. However, in recent years, in order to accelerate the purification of exhaust gas by utilizing a chemical property of the support, it has been proposed to use various other materials such as ceria (CeO2), zirconia (ZrO2) and/or titania (TiO2) in combination or not in combination with alumina.
For example, in order to enhance the exhaust gas purifying ability of the three-way catalyst by absorbing the fluctuation of oxygen concentration in the exhaust gas, a material having an oxygen storage capacity (OSC) of storing oxygen when the oxygen concentration in the exhaust gas is high, and releasing oxygen when the oxygen concentration in the exhaust gas is low, is used for an exhaust gas purification catalyst. A representative material having oxygen storage capacity is ceria.
For achieving efficient progress of oxidation of CO and HC and reduction of NOx by the action of a three-way catalyst, the air-fuel ratio of an internal combustion engine needs to be a theoretical air-fuel ratio (stoichiometric ratio), and therefore maintaining the oxygen concentration around the theoretical air-fuel ratio by causing a material having an oxygen storage capacity to absorb the fluctuation of oxygen concentration in the exhaust gas is preferred in order for the three-way catalyst to exert the exhaust gas purifying ability. Furthermore, according to recent studies, it has been found that ceria has not only an oxygen storage capacity but also strong affinity for a noble metal, particularly platinum, supported thereon, and therefore can suppress the grain growth (sintering) of the noble metal.
Thus, ceria has a preferable property with respect to use in an exhaust gas purification catalyst but sometimes does not have heat resistance required in this application. Accordingly, it has been proposed to form a ceria-zirconia-based composite oxide by dissolving ceria and zirconia in solid and thereby enhance the heat resistance.
For example, Patent Document 1 discloses an oxygen storage material containing a ceria-zirconia-based composite oxide, wherein a pyrochlore-type regular array phase (hereinafter, referred to as “ceria-zirconia-based regular array phase”) is formed by a cerium ion and a zirconium ion in the ceria-zirconia-based composite oxide.
The ceria-zirconia-based regular array phase is a pyrochlore phase in a reduced state and a κ phase in an oxidized state. The pyrochlore phase has an oxygen-deficient site and, upon introduction of an oxygen atom into the site, the pyrochlore phase transforms into a κ phase. On the other hand, the κ phase transforms into the pyrochlore phase by releasing the oxygen atom. The oxygen storage capacity of the ceria-zirconia composite oxide is realized by absorbing and releasing oxygen during reciprocal phase transformation between the pyrochlore phase and the κ phase.
In the case of using a ceria-zirconia-based composite oxide as an oxygen storage material for an exhaust gas catalyst, the ceria-zirconia-based regular array phase formed in the ceria-zirconia-based composite oxide transforms into a pyrochlore phase in a fuel-rich state and turns into a κ phase in a fuel-lean state. Such a phase transformation is produced by the entering and leaving of oxygen in the surface of the ceria-zirconia-based regular array phase.
When the particle diameter of the ceria-zirconia-based regular array phase is large, oxygen can hardly enter and leave the inside of the ceria-zirconia-based regular array phase. As a result, the oxygen storage speed of the ceria-zirconia-based regular array phase lowers, and the oxygen storage temperature rises.